1. Field of Invention
The invention hereinafter described and claimed pertains to actuators which are used to convert fluid pressure to mechanical movement. More particularly, this invention pertains to such actuators which produce rotary motion by means of an annular piston or pistons.
2. Prior Art
Rotary actuators are used to open and close doors or windows, raise and lower flaps on an airplane wing, turn switches or operate valves and any other device which must be moved in rotary fashion about a center point. Such actuators are generally fluid driven, meaning that a fluid, typically hydraulic or pneumatic, is used to cause the rotary movement.
One mechanism used to obtain this rotary movement utilizes one or more annular pistons. This is old in the art. For example, U.S. Pat. No. 163,186, issued May 11, 1875, discloses an oscillating engine in which a pair of opposing annular pistons are utilized to generate motion. A similar arrangement of oscillating dual annular pistons is more recently disclosed in U.S. Pat. No. 3,446,120, issued Nov. 14, 1966, in an oscillating fluid-driven actuator.
Methods other than the annular piston arrangement have been used in the past to produce this rotary motion. A straight piston mechanism, in which the linear movement of the piston moves a rack gear across a gear shaft, imparting rotary motion to the gear shaft, has been used. The annular piston arrangement, however, produces greater torque and is more compact and light weight.
Actuators may be single acting or double acting. In a single acting actuator, motion is produced in one direction only. For example, in a single piston mechanism, the piston is caused to travel in the piston chamber by the input of fluid pressure medium into the chamber. Once the pressure is released, the piston will be returned to its starting position, usually by the gravitational weight of the device which was originally moved by the piston or by a spring. In a double acting actuator, rotary motion is produced in both the counterclockwise and clockwise directions. This is shown, for example, in U.S. Pat. No. 3,446,120, in which opposing annular segment shaped pistons, working in similarly opposing piston chambers, can be made to move in opposing directions by the oscillation of pressure to the opposing chambers. Obviously, there are many uses for an actuator where the double acting feature will be preferred, if not required.
The prior art rotary actuators have suffered from a number of drawbacks. The first drawback arises as a result of the difficulty in providing for a sufficiently durable seal between the piston head and the annular chamber. Because of the variances in the dimensions of the annular chamber which can arise during manufacture, difficulty was encountered in designing a piston head which would provide a sufficiently durable seal against the walls of the annular chamber during operation, without creating excessive friction between the piston head and the wall. From a design standpoint it is the ultimate goal to produce the maximum torque from the rotary actuator for a given chamber pressure. It will be easily understood that any excess friction between the piston head and the walls of the annular chamber will greatly reduce the resultant torque from a given chamber pressure.
A further drawback encountered with the annular piston arrangement was that under any substantial pressure, the annular piston arm will undergo some flexing as the direction of the force exerted on the piston arm by the fluid pressure against the piston head is linear and is tangential to the axis of the piston arm. Any slight flexing of the piston arm would cause increased friction between the piston head and the wall of the annular chamber thereby reducing performance of the actuator.
One method which has been utilized to attempt to neutralize this friction buildup resulting from deformations of the chamber wall and of the piston arm is to allow the piston head to "float" relative to the piston arm. This is shown, for example, in Sneen, U.S. Pat. No. 3,446,120 (see FIG. 11 of U.S. Pat. No. 3,446,120) and in Mehm, U.S. Pat. No. 2,649,077 (see FIG. 2 of U.S. Pat. No. 2,649,077). This method aided in the reduction of friction, but there was still considerable friction generated in these devices by the friction of the piston head against the piston arm. In other words, when the piston head is under substantial pressure, it is forced, under that great pressure, against the piston arm. Accordingly, there will be great friction between the piston head and the piston arm should the piston head attempt to move laterally relative to the piston arm. Therefore, the fact that the piston head is designed to "float" in the prior art devices does not improve the performance of the actuator as well as it might because the friction between the piston head and the piston arm will tend to hold the piston head against the chamber wall with greater force, thereby producing unnecessary friction between the piston head and the chamber wall.
Another method used to reduce sidewall friction has been the installation of slide pads of antifriction material on the exterior side of the piston arm which abuts the chamber wall. See Sneen, U.S. Pat. No. 3,444,788, issued May 20, 1969. This method also does not address the problem of piston head/piston arm friction.
Another drawback inherent in the prior art devices resulted from the cumbersome and often complex design and relationship of the component parts resulting in higher costs of manufacture and maintenance and lower performance.